Method for recovering propane and heavier hydrocarbons



F. w. WINN ET AL 2, 6,019 RECOVERING PROPANE AND HEAVIER HYDROCARBONS Jan. 12, 1954 METHOD FOR Filed July 28, 1950 v METHANE AND ETHANE FRANCIS w. WINN LESTER R. STEFFENS INVENTORS ATTORNEY OR AGENT Patented Jan. 12, 1954 f 2,6t6,tl9 osrreo STATES PATENT orrice METHOD FOR RECOVERING PROPANE AND HEAVIER HYDROCARBONS Francis W. Winn, Woodbury, and Lester R. Steffens, Pitman, N. J., assignors to Socony- Vacuum Oil Company, Incorporated, a corporation of New York Application July 28, 1950, Serial No. 176,286

3 Claims. (01. 196-11) 1 2 This invention relates to a process for reshown is a schematic View of the apparatus emcovering liquids and gases from petroleum fracployed in the instant process. tions. In particular, the method described here- Referring more particularly to the drawing, in is concerned with an improved procedure for numeral Ill designates a conduit for the introrecovering propane and heavier hydrocarbons duction of hydrocarbon feed, which is generally from a petroleum fraction boiling within the a mixture of hydrocarbons boiling within the range of gasoline by effective demethanization range of gasoline. The gasoline fraction so inand deethanization thereof. troduced is suitably the eflluent of a reformer The conventional processes heretofore emoperating at high pressure. The vaporous hyployed for effecting removal of methane and drocarbon charge is quenched in vessel H, and ethane from petroleum fractions containing hythereafter further cooled and condensed in redrocarbons boiling within the range of gasoline boiler service by passage through pipe l2, through have involved feeding the stock to be treated into reboilers I3 and M, and then via pipes and it a bubble tower maintained at a temperature such through an additional cooler ll. The cooled that the methane and ethane tend to pass over- 15 charge is then recycled, as shown, to the original head while the higher boiling hydrocarbons are feed line H] serving to at least partially cool obtained as a bottoms product. In practice, the incoming hydrocarbon charge. The cooled however, such processes, due to lack of ideal confeed finally passes from vessel I I through contaming Conditions, ve i v ab y resulted in duit I8 and is totally condensed and water-cooled the passage overhead of considerable amounts upon passage through cooler IS. The cold feed of propane, butane, and heavier hydrocarbons then flows through pipe 20, condenser 21, and along with the methane and ethane whose repipe 22 into reflux accumulator 23 and thereafter moval was desired. Subsequent recovery of propasses from said accumulator through conduits pane and heavier hydrocarbons from the over- 24 and 25 and pump 26, the latter serving to force head has generally entailed passage of the same 25 the cold feed through pipe 27 and into the upper through an absorber-stripper system. Such portion of fractionating column 28. This column operation, however, is relatively expensive. Very is operated essentially as a high pressure strip" often, it has accordingly been the practice to per and it is important for present operation that forego high recovery of propane and heavier hythe feed be introduced into the column at the drocarbon components from the overhead with point of maximum pressure and minimum temthe result that substantial losses of these maperature in order to effectively reduce the overterials were incurred. head loss of propane and other desired heavy It is an object of the present invention to recomponents. This is accomplished in accordance duce the aforementioned losses by providing a with the instant method by introducing the feed method for recovering substantially all of the into the high pressure column via the reflux propane, as well as heavier hydrocarbons, from condenser or alternatively, as discussed hereina petroleum fraction containing hydrocarbons below, to the top plate of the condenser. boiling within the range of gasoline. It is a fur- The overhead gas from high pressure column ther object of this invention to accomplish the 28 passes through outlet pipe 29 into line 29 car" removal of methane and ethane from such frac- 4O rying the cold liquid feed, This o bi d tions without simultaneously removing propane stream is conducted through condenser 21 and and heavier hydrocarbons therefrom. A still fur- P p 22 into accumulator The Overhead gas ther important object is the provision of an imflows from alccllmulatol 23 through Conduits 33 proved process for economically elfecting deand to turblne 32 wherein it P ded to ethanization and demethanization of a petroleum fuel 11116 p e ure (about 30p. 5. 1.). The as fraction with minimum accompanying loss of becomes colddurmgfixpanslon g d flows from butane and higher hydrocarbons. turbme 32 through E 33 and ough repropane, flux condenser 2| serving to cool the same. The These and other ob ects which W111 be apoverhead of ethane an d h H ghter gases then passes parent to those skilled in the art are attamed 1n thmu 1 A 5o gh pipe 35 and may be used as a fuel gas accordance with the procedure of the instant mor for any other desired purpose. vention wherein petroleum fractions contam- Since the loss of propane and heavier coming hydrocarbons bo Wlth1n the range of ponent is related to the temperature of the reflux gasoline are essentially demethanized and decondenser, it is desirable to minimize the loss by ethanized by passage of the Same through a maintaining the condenser at a temperature as combination of two fractionating columns, one of l as consistent with commercial operation which is operated at a high pressure and the The auto-refrigeration obtained through expanother of which is operated at a considerably sion of overhead gases as described above may, lower pressure. if desirable, be augmented by additional re- In the accompanying drawing, the figure frigeration. On the other hand, if hydrate for- 3 mation is considereda problem, the temperature could be'maintainedat a point ofiabout 70 F: or higher, at which no hydrate formation would be encountered.

The bottoms product from high pressure column 28 passes through outletconduitflF -tb thetop tray of fractionating column 3T'wh'iclris operated at an appreciably. "lower pressurerthan column 28. Deethanization ofi the feed-change: is completed in this low pressure column- The overhead vapor issuing from this column consists of ethane saturated with heavier components and is conducted to the bottomofhigh' pressure column 28 in order to recover the propane and butane contained therein and to provide heat necessary for the stripping cper at'ion accomplished in the high pressure-columns The recycling from column 3"|"tocolumn- 28'- is effected by passage'of the overhead through conduit 38 through cooler 39and thenintoreflux acccumulator 4131 The recycle. vapor passesirom accumulator dli'through pipe illinto aatur-bocompressor -32' driven by overheadigas from col.- umn 28, which gas is conducted-from the reflux accumulator 23 of said column through conduit 3i and turbine 32; The gas'int-roducedli'nto turbo-compressor 4'2" from accumulator I hecomes heated during. compression thereof: and the resulting hot gas under pressure is 'led' through conduit '43into the bottom oficolumn 23';

Reflux from accumulator W is returned-"to column '31 via conduit 4'4, the flow being. aided by pump 45. The bottoms from lowpressure column 37' are removed through outlet ,pip'e 4'6. The bottoms so obtained contain tar; gasoline, butane, propane; and'less than" 1%" of ethane (based on propane content) This material thus substantially free of ethane and lighter components may thereafte'rb'e fractionated intWth'e various products by conventional means It is to'be realized that'temperatures'andpres sures employed in variousparts of the forego-- ing system are of extreme importance in accomplishing the stated objects 'of'*this"'mvention; The term convergence pressure as used herein is the maximum pressure, for" any specifiedtemperature; at which any separation of"thecomponents of the particular hydrocarbon minture can beaccomplishedby distillation. As this' pressure is approached, the vapor=liouid *equi-- librium ratios of all thecomponerits-[converge toward unity; all'relative volatilit'ies'of the-"com ponents approach unity; and" the difificulty of separation increases; At the--convergen"ce=p1'essure, theequilibrium-ratios"are all equal "to unity;

relative volatility likewise'isunity"forall com ponents, and separating isimpo'ss'iblie; Thefeed" charge, as pointed outhereinabcve; 'isprereramy, but not necessarily, the efiiuent -of -a high pres sure reformer. In such instances; the pressureof the feed, and consequently the pressureat which column 28 is operated, will correspond substantially to the full outlet pressure 'of-" the reformer. Under the usual conditions of the instant process, the minimumpressureaiiwhich column 28 is operated is about 200 p'oundsper square inch lessthan the convergencepres'sum of the bottoms product-cfcolumn 33. As' a general rule, the pressure at whichcolumn-2B is? operated is within the approximate range-300 to 3000 pounds per square inch and preferablybetween about 500 and about 2000 pounds 1331i square inch. Particularlyf'efficient operatiomis' accomplished in the-vicinityot aboutlmfio pounds per square inch. The temperature ofi the fed; assuming it to be the efiiuentof a high pressure 75 reformer, is generally about. 1000 F. This elev'ated temperature is considerably lowered by quenching and cooling of the feed to an extent such that the temperature thereof after passing through cooler 19 is F. or less. The temperatureof the feed introduced into column 28 generallyfurther reduced upon passing through condenser: 24. Indeed, the losses of propane. -butane,,.and heavier components by passage ofethese gases overhead from column 28 are-relatively sm'alldf the reflux condenser temperature of. this column is maintained below about 100" F. The losses at varying temperatures for a 10,000 barrel per day reformer gas plant are set forth in the table below:

i r 'Lcss Perce t on. Loss (Barrels per day) 1 Reflux Condenser Component Temp,

Propane Butane 05+ .PropaneButane 10o a. 42 is s. 1 3.0 60; 58* 18; 6 358' 1:3 40 32 9 v 3 Y 2.1. 017

'From the above,,it' will be apparent that. the" temperature at whichithe .feed is introducedinto the high pressure fraction'atirig-columngenerally should not exceed about 100FI' Preferably, this temperature should be 60" F. or less in order to minimize the loss of desired" propane andheavier components".

It' is" further important in practice of the present. invention that'the' feed be introduced into the 'high' pressure column at the point of lowest temperature; In accordance with conventional fractionating tower operation, feed charged thereto suoj ected to" a stripping action by raising its temperature as. it flows. down the column and'the vaporsthus stripped have their:

' temperature lowered astheyirise"irr'the-"column.

The point at whici'r'feed is'intro'duced into the column of the present invention is'the'top of column where-the temperatureisata minimum; This is accomplished; in accordance'with the instant procedure; by introduction ofth'e feed to the-refiux condenser oftl'ie highpressure'c'olumn' or, alternatively, the feed, after-being" cooled; may'beintroduced to the-top *pla't'eof the condenser byway'of conduiM-li The bottomstemperature of the second" col-- umn is maintained-high enoughto insure-"boil ingof' the bottoms product at"the'-eXistent pres-- sure and to reject all ethane-and lower boiling components as overhead. This bottoms "tem pera't-ure generally is within the range 300 to- 450 depending upontheparticular pressure conditionsexistentin the -column. This pres sure, in turn' isi established atva level sushi that" critical -.conditions oiithei desiredbottomsgprcduct: will: not be encountered in the? bottom: of: the column. Ordinarily, the pressure at. whichthis second column is operated shouldrbe 500 pounds per square inchor less. Thepressure however, should not be too low or otherwise auxiliary power will be requii ed for operationcfthe turbocompressor. The minimum feasible pressurefor effective operation" 01" this column is generally about 200 pounds per square inch; It is fur* ther desirable in operating the instantly-de scribed system that the 1 pressure difierentiailbe-- tween the high pressure column 28? and? the low pressure column 31 be atleast 1 about 200' pounds per. square: inch andxsuita-bly about 600 pounds. per square inch. Thus, it is preferred to'cperate the upper high pressure: fractionating c01- umn at about 1000 pounds per square inch and the low pressure column at about a pressure of about 400 pounds per square inch.

With the foregoing combination of fractionating columns, effective demethanization of a gasoline fraction is carried out while reducing to a minimum the losses of propane and heavier hydrocarbon components heretofore encountered. Thus, in accordance with the present invention, substantially all of the propane, as well as heavier components, are recovered from a petroleum fraction containing hydrocarbons boiling within the range of gasoline while accomplishing almost complete rejection of methane and ethane from said fraction.

It is to be understood that the above description is merely illustrative of preferred embodiments of the invention of which many variations may be made within the scope of the following claims by those skilled in the art without departing from the spirit thereof.

We claim:

1. A method for removing methane and ethane from a petroleum fraction containing hydrocarbons boiling within the range of gasoline, which comprises introducing said petroleum fraction into a high pressure fractionating zone at the point of minimum temperature therein, passing the overhead gas emitted from said high pressure zone to a condensin zone wherein high boiling components of said overhead gas are liquefied, recycling said liquefied components as reflux to I said high pressure fractionating zone, expanding remaining gas with resultant cooling thereof, flowing the resulting cooled gas in indirect heat exchange with the aforementioned condensing zone serving to cool the same, passing the liquid bottoms product of said high pressure fractionating zone into a second fractionating zone maintained at a pressure of at least 200 pounds per square inch less than the pressure of said high pressure fractionating zone, the minimum pres sure of said second fractionating zone in turn being about 200 pounds per square inch, withdrawing the vaporous efiiuent of said second fractionating zone, cooling the same to liquefy the high boiling components thereof, recycling resulting liquid as reflux to said second fractionating zone, compressing remaining vapor in a turbocompressor driven by the aforesaid overhead gas from said high pressure zone, conducting the resulting gas, heated as a result of compression, to the bottom of said high pressure fractionating zone, withdrawing methane and ethane as overhead from said high pressure fractionating zone and withdrawing heavier hydrocarbons as bottoms product from said second fractionating zone.

2. A method for removing methane and ethane from a petroleum fraction containing hydrocarbons boiling within the range of gasoline, which comprises introducing said petroleum fraction at a temperature not exceeding about 100 1". into a high pressure fractionating zone at the point of minimum temperature therein, said zone being maintained at a pressure between about 300 and about 3000 pounds per square inch, passing the overhead gas emitted from said high pressure zone to a condensing zone wherein high boiling components of said overhead gas are liquefied, recycling said liquefied components as reflux to said high pressure fractionating zone, expanding remaining gas with resultant cooling thereof, flowing the resulting cooled gas in indirect heat exchange with the aforementioned condensing zone serving to cool the same, passing the liquid bottoms product of said high pressure fractionating zone into a second fractionating zone maintained at a pressure of at least 200 pounds per square inch less than the aforementioned high pressure zone, the pressure of said second fractionating zone being between about 200 and about 500 pounds per square inch, withdrawing the vaporous efiiuent of said second fractionating zone, cooling the same to liquefy high boiling components thereof, recycling resulting liquid as reflux to said second fractionating zone, compressing remaining vapor by the pressure action of the aforesaid overhead gas from said high pressure zone, conducting the resulting gas, heated as a result of compression, to the bottom of said high pressure fractionating zone, withdrawing methane and ethane as overhead from said high pressure fractionating zone and withdrawing heavier hydrocarbons as bottoms product from said second fractionating zone.

3. A method for removing methane and ethane from a petroleum fraction containing hydrocarbons boiling within a range of gasoline, which comprises introducing said petroleum fraction into a high pressure fractionating zone at a temperature not exceeding F. at the point of minimum temperature therein, said zone being maintained at a pressure between about 500 and about 2000 pounds per square inch, passing the overhead gas emitted from said high pressure zone to a condensing zone wherein high boiling components of said overhead gas are liquefied, recycling said liquefied components as reflux to said high pressure fractionating zone, expanding remaining gas with resultant cooling thereof, flowing the resulting cooled gas in indirect heat exchange with the aforementioned condensing zone serving to cool the same, passing the liquid bottoms product of said high pressure fractionating zone into a second fractionating zone operated at a pressure between about 200 and about 500 pounds per square inch and maintained at a temperature sufiicient to reject ethane and lower boiling components, the minimum pressure differential between said high pressure fractionating zone and said second fractionating zone being between about 200 pounds per square inch, withdrawing the vaporous efi'iuent of said second fractionating zone, cooling the same to liquefy high boiling components thereof, recycling resulting liquid as reflux to said second fractionating zone, compressing remaining vapor by the pressure action of the aforesaid overhead gas from said high pressure zone, conducting the resulting compressed gas, heated as a result of compression, to the bottom of said high pressure fractionating zone, withdrawing methane and ethane as overhead from said high pressure fractionating zone and withdrawing heavier hydrocarbons as bottoms product from said second fractionating zone.

FRANCIS W. WINN. LESTER R. STEF'FENS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,072,456 Keith Mar. 22, 1937 2,231,444 Gerhold Feb. 11, 1941 2,277,387 Carney Mar. 24, 1942 7 2,327,896 Houghland Aug. 24, 1943 2,342,165 Plummer Feb. 22, 1944 2,487,147 Latcham Nov. 8, 1949 

1. A METHOD FOR REMOVING METHANE AND ETHANE FROM A PETROLEUM FRACTION CONTAINING HYDROCARBONS BOILING WITHIN THE RANGE OF GASOLINE, WHICH COMPRISES INTRODUCING SAID PETROLEUM FRACTION INTO A HIGH PRESSURE FRACTIONATING ZONE AT THE POINT TO MINIMUM TEMPERATURE THEREIN, PASSING THE OVERHEAD GAS EMITTED FROM SAID HIGH PRESSURE ZONE TO A CONDENSING ZONE WHEREIN HIGH BOILING COMPONENTS OF SAID OVERHEAD GAS ARE LIQUEFIED, RECYCLING SAID LIQUEFIED COMPONENTS AS REFLUX TO SAID HIGH PRESSURE FRACTIONATING ZONE, EXPANDING REMAINING GAS WITH RESULTANT COOLING THEREOF, FLOWING THE RESULTING COOLED GAS IN INDIRECT HEAT EXCHANGE WITH THE AFOREMENTIONED CONDENSING ZONE SERVING TO COOL THE SAME, PASSING THE LIQUID BOTTOMS PRODUCT OF SAID HIGH PRESSURE FRACTIONATING ZONE INTO A SECOND FRACTIONATING ZONE MAINTAINED AT A PRESSURE OF AT LEAST 200 POUNDS PER SQUARE INCH LESS THAN THE PRESSURE OF SAID HIGH PRESSURE FRACTIONATING ZONE, THE MINIMUM PRESSURE OF SAID SECOND FRACTIONATING ZONE IN TURN BEING ABOUT 200 POUNDS PER SQUARE INCH, WITHDRAWING THE VAPOROUS EFFLUENT OF SAID SECOND FRACTIONATING ZONE, COOLING THE SAME TO LIQUEFY THE HIGH BOILING COMPONENTS THEREOF, RECYCLING RESULTING LIQUID AS REFLUX TO SAID SECOND FRACTIONATING ZONE, COMPRESSING REMAINING VAPOR IN A TURBOCOMPRESSOR DRIVEN BY THE AFORESAID OVERHEAD GAS FROM SAID HIGH PRESSURE ZONE, CONDUCTING THE RESULTING GAS, HEATED AS A RESULT OF COMPRESSION, TO THE BOTTOM OF SAID HIGH PRESSURE FRACTIONATING ZONE, WITHDRAWING METHANE AND ETHANE AS OVERHEAD FROM SAID HIGH PRESSURE FRACTIONATING ZONE AND WITHDRAWING HEAVIER HYDROCARBONS AS BOTTOMS PRODUCT FROM SAID SECOND FRACTIONATING ZONE. 